Einstein s theory of special relativity

Transcription

1 Einstein s theory of special relativity Announcements: Homework 1s due at 1:00pm on Friday in the wood cabinet just inside the physics help room (G2B90) Last year s Nobel Prize winner David Wineland (CU and NIST) is giving the colloquium at 4pm in G1B20. As last week there are cookies at 3:45pm. Albert Einstein ( ): Today we will derive time dilation (and maybe length contraction) Physics 2170 Fall

2 For Colloquium:Schrödinger's Cat A cat is placed into a steel chamber, along with a device containing a vial of a radioactive substance. If a single decay occurs during the test period, a relay mechanism will break the vial and kill the cat. The observer cannot know whether or not an atom of the substance has decayed, and consequently, cannot know whether the vial has been broken, the released, and the cat killed. Since we cannot know, according to quantum mechanics, the cat is both dead and alive, in what is called a superposition of states. Only when we break open the box and learn the condition of the cat is the superposition is lost, and the cat becomes one or the other (dead or alive). This is called quantum indeterminacy or the observer's paradox: the observation or measurement itself affects an outcome. (There is no single outcome unless it is observed.) Physics 2170 Fall

3 What is the plot? Time (days) 11/04/10 The Large Hadron Collider 3

4 Today: Clocks run at different rates in different reference frames By how much? Proper time We are going to ditch trains and move on to faster moving objects. Examples are often space ships where we can have large effects. Time Dilation Physics 2170 Fall

5 Use Light Clock on a spaceship A light flash starts a tick and when the light reflects off the mirror and back down to a detector it fires off another light flash. So a light clock ticks each time light makes a complete circuit: up and down. For Lucy, how long does each tick take? Δt L = 2d/c This is the time between two events: Event 1: The light flashes Event 2: The reflected light reaches the detector Physics 2170 Fall

6 Lucy s spaceship flies by Earth at speed v Ricky is on Earth watching Lucy s spaceship fly by at a speed v. Ricky has observers set up at the location of the two events. Using their synchronized clocks he finds that a time of Δt R passes between the light flash and when the reflected light reaches the detector. Physics 2170 Fall

7 Clicker question 1 Set frequency to AD Pythagorean theorem (X2 for two triangles) According to Ricky, how far does the light beam travel between the two events (light flash and reflected light hitting the detector)? A. B. C. D. E. None of A-D Physics 2170 Fall

8 Clicker question 1 Set frequency to AD Pythagorean theorem (X2 for two triangles) According to Ricky, how far does the light beam travel between the two events (light flash and reflected light hitting the detector)? A. B. C. D. E. None of A-D Note, this length is also equal to c Δt R Physics 2170 Fall

9 Algebra to give us the time dilation formula Ricky sees light travel a distance of The distance traveled by light can also be written: Squaring both sides gives Collecting terms gives Dividing both sides by gives Dividing by square root gives and taking the Note the time measured by Lucy is Therefore, the relation between the time observed by Ricky (on Earth) and Lucy (in the spaceship) is: Physics 2170 Fall

10 Clicker question 2 Time Dilation The relationship between the time between two events in Ricky s frame (on Earth) and the time between the same two events in Lucy s frame (on spaceship) is: Define and so then Q. According to Ricky, the time between the two events (light flash and reflected light reaching detector) is A. greater than what is observed by Lucy B. less than what is observed by Lucy C. equal to what is observed by Lucy Physics 2170 Fall

11 Clicker question 2 Time Dilation The relationship between the time between two events in Ricky s frame (on Earth) and the time between the same two events in Lucy s frame (on spaceship) is: Define and so then Q. According to Ricky, the time between the two events (light flash and reflected light reaching detector) is A. greater than what is observed by Lucy B. less than what is observed by Lucy C. equal to what is observed by Lucy and therefore So. They are equal only when v = 0. Physics 2170 Fall

12 Compare the two frames There is something special about the two events in Lucy s frame! Both events occur at the same location in Lucy s frame. Physics 2170 Fall

13 Proper time and time dilation If two events occur at the same location, then the time between them can be measured by a single observer with a single clock (the Lucy time in our example). The time measured between these types of events is called the proper time, Example: any given clock never moves with respect to itself. It keeps proper time in its own frame. Any observer moving with respect to this clock sees it run slowly (i.e., time intervals are longer). This is time dilation: Physics 2170 Fall

14 Clicker question 3 Set frequency to AD v Vicki Carol Q. Carol and Vicki are identical twins. While Vicki stays on Earth, Carol departs for the star Sirius, 8 light-years away, traveling at a speed v = 0.8c (Note γ = 5/3). According to observers in Vicki s frame, how long does the trip take? A. 6 years B. 8 years C. 10 years D years Physics 2170 Fall

15 Clicker question 3 Set frequency to AD v Vicki Carol Q. Carol and Vicki are identical twins. While Vicki stays on Earth, Carol departs for the star Sirius, 8 light-years away, traveling at a speed v = 0.8c (Note γ = 5/3). According to observers in Vicki s frame, how long does the trip take? A. 6 years B. 8 years C. 10 years D years The distance is 8 light-years and the speed is 0.8 c so we get Physics 2170 Fall

16 Clicker question 4 Set frequency to AD Vicki Carol Q. Vicki stays on Earth and Carol departs for the star Sirius, 8 light-years away, traveling at a speed v = 0.8 c (γ = 5/3). According to Carol, how long does the trip take? A. 6 years B. 8 years C. 10 years D years v Physics 2170 Fall

17 Clicker question 4 Set frequency to AD Vicki Q. Vicki stays on Earth and Carol departs for the star Sirius, 8 light-years away, traveling at a speed v = 0.8 c (γ = 5/3). According to Carol, how long does the trip take? A. 6 years B. 8 years C. 10 years D years Solving for the proper time gives Carol Carol s clock is the only one present at both events (leaving Earth and arriving at Sirius). Therefore, she is measuring the proper time! This tells us which time is which in the time dilation formula. v Follow the proper time! Physics 2170 Fall

18 Length contraction v Vicki Carol We found that only 6 years pass in Carol s reference frame as she travels 8 light-years to Sirius at 0.8 c. We say that every inertial frame is valid so won t she observe the space ship going faster than the speed of light (8 light years in 6 years!)? This discrepancy is resolved by length contraction moving objects are shorter (in the direction of motion) Physics 2170 Fall

19 Time Dilation How come we never notice the effect of time dilation? How much time will your watch lose on a non-stop flight from Los Angeles to Sydney (11 hours at 300 m/s)? Time dilation is If our watch measures 11 hours, the time recorded on the ground will be longer: = 11 hours + 20 ns Not usually noticeable but in fact measured in 1971 by carrying 4 atomic clocks onboard a jet airplane around the world + synchronized to a reference clock. Found 275 +/- 7 ns discrepancy with reference clock. Physics 2170 Fall

20 Example Muon Lifetime Muon has a mean lifetime of 2.2µs when at rest. What is the mean lifetime if it is traveling at speed 0.6c = 1.8 x 10 8 m/s? Answer is Δt = Δt 0 1 v 2 c 2 = s Most muons passing through us have a much higher speed and thus live much longer. Physics 2170 Fall

21 Length contraction v=0.8c Vicki Carol 8 light-years We found that only 6 years pass in Carol s reference frame as she travels 8 light-years to Sirius at 0.8 c. We say that every inertial frame is valid so won t she observe the space ship going faster than the speed of light (8 light years in 6 years!)? This discrepancy is resolved by length contraction moving objects are shorter (in the direction of motion) Physics 2170 Fall

22 Length contraction Carol sees a star approaching at 0.80c which gives and measures the length between Earth and star as So in Carol s frame the ship covers 4.8 light years in 6 years which works out to a velocity of 4.8/6 = 0.8c, same as the velocity seen on earth by Vicki. Physics 2170 Fall

23 Length of an object This stick is 3m long. I measure both ends at the same time in my frame of reference This length, measured in the stick s rest frame, is its proper length. Or not. It doesn t matter, because the stick isn t going anywhere. But as we know, at the same time is relative it depends on how you re moving. Physics 2170 Fall

24 Length of an object S S 0 v Another observer comes whizzing by at speed v. This observer measures the length of the stick, and keeps track of the time. Event 1 Origin of S passes left end of stick. Physics 2170 Fall

25 Length of an object S S v Event 1 Origin of S passes left end of stick. Event 2 Origin of S passes right end of stick. How many observers are required to measure the time between these two events in reference frame S and S? 0 Physics 2170 Fall

26 Length of an object S S v Event 1 Origin of S passes left end of stick. Event 2 Origin of S passes right end of stick. How many observers are required to measure the time between these two events in reference frame S and S? For S: 2 observers at 0 and 3. 0 For S : just one at the origin Physics 2170 Fall